An OxyChem representative said the increase is based largely on the falling price in recent weeks of polyvinyl chloride (PVC), a co-derivative of the electrolysis process.

As PVC prices fall, producton of chlorine is likely to fall in the coming weeks, creating tighteness in the availablity of caustic, the source said.

Formosa and Shintech did not immediately provide a reason for the increase.

The three announcments follow one made in early April by US producer Dow of $50/dst for May contracts. But buyers have firmly resisted that increase.

April contract prices are assessed at $470-540/dst.

A market participant said that the announcements, including the one made earlier by Dow, could be an attempt to buttress pricing in the face of downward pressure from buyers in recent weeks. However, the source said, there is a strong likelihood that the latest announcements will be implemented.

Major US caustic soda producers include Formosa, Shintech, Dow Chemical, PPG and Olin.